High frequency electrical oscillators



Oct. 9, 1956 J. F. sKowRoN 2,766,403

HIGH FREQUENCY ELECTRICAL osCILLAToRs Filed June 1952` UHU /NVENTOR y JOHN l-T SKow/QON AT NES/ J. F. SKOW RON HIGH FREQUENCY ELECTRICAL osCILLAToRs 141: /4m /4n /4p Oct.'9, 1956 Filed June 14, 1952 United States Patent O 2,766,403 HIGH FREQUENCY ELECTRICAL oscILLAToRs John F. Skowron, Waltham, Mass.,

Manufacturing Company, tion of Delaware assignor to Raytheon Newton, Mass., a corpora- 'lhis invention concerns a vhigh frequency electrical oscillator of the magnetron type and, more specically, to a strapped magnetron for producing the proper loading of both doublets or" the mode adjacent to the desired or pi mode.

Strapped magnetrons should have an anode resonant system such that the tube is capable of stable operation 1n the principal of pi mode of resonance. Stable operation in the desired mode is complicated owing to the existence of a comparatively strong adjacent mode which may be excited by approximately the same electronic operating condition.

Furthermore, the adjacent mode is a doublet; that is, it consists of two resonances whose field patterns contain the same number of angular variations around the cylindrical anode structure but differ by ninety electrical degrees in the angular position of the pattern maxima and minima. Because of an asymmetry introduced into the anode resonant system, such as may be obtained by the usual output coupling device attached to a single cavity of the anode system, the doublets are so oriented that one is coupled to the output system while the other 1s not.

Since the ratio of build-up of radio frequency energy in the magnetron cavity is directly proportional to theV Q of the resonance, it is desirable, in order that the pi mode be the dominantmode, that the Qs ofthe undesired resonance be less than that of the pi mode. With anodes containing no asymmetry other than that owing' to the output-coupling means, this condition does not exist, since one doublet of the adjacent mode remains uncoupled to the output system.

Pursuant to this invention, the degree of coupling between the doublets of the adjacent mode and the output of the strapped magnetron is controlled by introducing an asymmetry or 'asymmetries in addition to the asymmetry owing to the output coupling means into the anode structure at points located on one or both critical load axes passing through the geometrical center of said magnetron and angularly displaced by a critical load angle from an output axis passing through the center of said output coupling means and the geometrical center of said magnetron. The critical load axes for a given magnetron are so arranged that asymmetries introduced thereon provide for equal coupling of both doublets of the mode or modes adjacent the pi mode to the output coupling means. This asymmetry is generally quite small and maybe produced by a small discontinuity, such as a bent strap, to change the natural resonant frequency by an amount of the order of one or twoper cent. v

The optimum coupling is determined by said critical load angle, which, for an otherwise symmetrical anode except for the output coupling means, will generally be forty-tive degrees.

In a perfectly symmetrical anode (except for the output-coupling means), it is necessary to bend straps only at any single forty-tive degreermultiple position on one of said critical loadV axes;v however, because of small practical construction asymmetries, it is desirable to bend the straps at at least four points to insure that no random asymmetries mask the eifectof those purposely introduced. Furthermore, the type of asymmetry introduced at two of the diametrically opposite points on one critical load axis should be of the reverse type from those introduced at the two diametrically opposite points on the other critical load axis. In other words, if an asymmetry of a kind that will tend to raise the frequency (reduction of strap conductance'or capacitance) is placed at +45 and +225 with respect to the output coupling means, an asymmetry tending to lower the frequency should be placed at the 45 and -225 positions. Otherwise, the effects of the asymmetries will neutralize one another and the output discontinuity will predominate and x the orientation of the doublets as it does in a symmetrical anode.

lf, however, the anode block contains suiliciently large asymmetries other than the output coupling asymmetry, such as imperfections in the anode block resulting from the manufacturing process, said critical load axes may depart somewhat from forty-live degrees with respect to the output axis. The type of external magnetron load also has a slight eifect upon the critical load angle at which optimum coupling of the two doublets is attained.

Furthermore, satisfactory operation in the pi mode has been found to exist in practice in certain types of magnetrons even though the loading of the two doublets of the adjacent mode is not exactly equal. For instance, it has been found that a departure of vas much as lifteen degrees from the theoretical critical load angle of fortyn five degrees may be made in some Vmagnetrons while still obtaining suflicient coupling of the adjacent mode doublets to t-he output coupling means to prevent operation in the undesired adjacent mode.

By means of this invention, the desired loading of the doublets may be obtained without causing excessive and undesirable changes in the adjacent mode frequencies and in separation of the adjacent mode from the pi mode, as is the case with magnetrons employing strap breaks or other drastic asymmetries. Secondly, this invention permits a loading of the doublets of the adjacent mode which is comparatively insensitive to frequency as compared with the frequency-sensitive adjustment of end cavity geometry for accomplishing doublet loading in fixed frequency tubes. Thus, this invention provides for practical and convenient loading of the adjacent mode doublet in a tunable magnetron.

In the drawings:

Fig. l is a transverse View, partly in section, of a multiple resonator magnetron according to the invention;

Fig. 2 is a development of the anode vanes showing certain strapping details; and

Fig. 3 shows the distribution of anode radio frequency potential for the pi mode and doublets of the adjacent mode of the resonator system shown in Fig. 1.

Fig. l shows a magnetron generally indicated by reference numeral 10 and comprising a cathode 11 and anode block 12 including a plurality of anode vanes 14a, 14b, 14C, and so forth, attached to a shoulder portion 13 of anode block 12.

A pair of concentrically-arranged closed annular straps 15 and 16 is shielded from the magnetron1 interaction space by being positioned in slots 17 and 17', arranged between the upper faces of anode vanes 14. These straps are preferably in the form of exible ribbons or Wires of an electrically-conductive material, such as copper or silver. Inner strap 15 is attached to the inner edges of slots 17, as by brazing, while outer strap 16 is similarly attached to the outer edges of slot 17', as shown `in Fig. 1. The alternate slots 17 and 17 are odset or staggered with respect to one another so that strap 15 is latented Oct. 9, 1956Y As shown in Fig. 2, the straps are'preferably secured to the top portion ofthe corresponding slots for reasons which will be apparent later.

Although the strapping shown in Fig. l is of they shielded double-ring type, the invention is not limited thereto and single-ring strapping, either shielded or unshielded, or echelon strapping may also be used. Furthermore, an annular strap may beV located in slots in opposite faces of anode vanes 14, as shown in U. S. Patent No. 2,550,641 to Spencer; To reduce undesirable capacitance effects between `surfaces of the straps and the cathode, a shielded strap, such as shown in Fig. l, is preferred to an Unshielded strap.

The invention, furthermore, is not limited to magnetrons havingresonators of the vane type. For example, any of the aforesaid types of strapping may be incorporated in a magnetron having either a slot-type cavity resonator, or a hole and slot type resonant structure, such as shown and described'in the aforesaid patent to Spencer. In the case of thehole and slot type magnetron, the resonators may be connected together by a pair of closed wire rings secured within annular grooves formed in one of the end faces of the anode block.

Referring again to Fig. l, the oscillations produced in the magnetron may be led out from the tube by means of an inductive coupling loop 25 (not shown in Fig. l) which may be inserted into one of the resonant cavities 18 through a radial bore 19 in anode block 12. The coupling means is conventional and is omitted from Fig. l of the drawing in order to simplify the description of the operation of the invention. A wave guide coupling means may be employed in lieu of the coupling loop.

The output axis 20 passing through the center of out put bore 19 and the geometrical center of the magnetron serves as axis of reference in so far as the rotational position of the anode field pattern is concerned. In magnetrons which are symmetrical except for the outputcoupling means positioned in or near one of the cavities, the coupling means serves to fix the rotational position of the field pattern.

The distribution of anode radio frequency potential for the pi mode and the adjacent doublet mode in the resonator system of Fig. l is shown in Fig. 3. The anode is shown developed from the cylindrical form with the various anode vanes appearing as rectangles. The output-coupling loop is represented by reference numeral 25. The full lines represent the potential variation with angular displacement from the asymmetry introduced by the output-coupling means on axis for the first doublet of the mode adjacent the pi mode. The dashed lines represent the potential variation associated with the second doublet of said adjacent mode with angle 9 measured from output axis 20.

The anode potential for the two doublets of the mode adjacent the pi mode, or the Ei l mode where N is the number of cavities, is shown by the wave patterns in Fig. 3a. For an anode having 16 cavities, such as the anode structure of Fig. l, said doublets are those of either the 11:7 or 11:9 mode where n is the order number of the mode. For the anode of Fig. l the pi mode is the 11:8 mode. The doublets of the n=7 mode are shown in Fig. 3.

It should be understood, however, that the invention is not restricted to an anode structure having 16 cavities; any even number of anode cavities may be used. From the wave forms of Fig. 3a, it is evident that the instantaneous voltage on anode segment or vane 14q for the first doublet is almost at the minimum value, as shown by point A. Similarly, the instantaneous voltage for the rst doublet at adjacent anode vane 14./1 is at approximately the same value as that at vane 14q but of reversed polarity, as shown at point B. There is, therefore, a considerable difference of potential between the anode segments bounding output cavities 18; the radio frequency current tiow along the output cavity vanes 14a and 14q and, hence, the magnetic ux in the output cavity is relatively large. The first doublet is thus strongly coupled to the output load of the magnetron. On the other hand, the instantaneous radio frequency voltages appearing at output vanes 14q and 14a for the second doublet are equal in magnitude and polarity, as shown at points C and D, respectively, so that there is no coupling of this second doublet to the output.

Since the oscillation in a loosely-coupled mode has a high Q because of its being dampened only by losses in the resonator structure itself, said oscillation will build up more rapidly under electron drive than that of the desired pi mode and this undesirable oscillation, either steady or intermittent, in the loosely-coupled doublet of the adjacent mode, occurs.

In order to eliminate the aforesaid difficulty inherent in unequal loading of the adjacent mode doublets, an arrangement as shown in Fig. 2 may be used. A development of the cylindrical'anode block is shown in Fig. 2 with the straps and anode vanes bearing corresponding reference numerals to those of Fig. l.

It should be understood that, although Fig. l illustrates single closed annular straps 15 and 16, it is possible that a plurality of separate straps may be soldered to alternate anode segments. For purposes of discussion and for establishing terminologyused in the claims, that portion of annular straps 15 and 16, which interconnects two alternate vanes or segments of the anode-resonant structure, will be referred to as a separate strap. In other words, straps 15 and 16 will eachbe considered as eight separate straps, all of which interconnect a pair of alternate vanes.

Referring back to Fig. l, the critical load axes 30 and 31 are each displaced from output axis 20 by a critical load angle of forty-five degrees. The straps which cross critical load axis 30 are pulled up, as shown in Fig. 2, thereby tending to increase the natural resonant frcquency of the cavities formed by the vanes to which these straps are attached. For instance, the inner strap 15 interconnecting vanes 14a and 14e and the outer strap 16 interconnecting vanes 14b and 14d are lifted up above the normal position, which, as here shown, is coincident with the top face of the anode vanes. Diametrically opposite the aforesaid straps another pair of straps crossing axis 30 is also pulled up; this pair includes the inner strap interconnecting anode vanes 14 and 14k and the outer strap connecting together vanes 14j and 141.

The straps which cross critical load axis 31 are pushed down, thereby tending to decrease the natural resonant frequency of the cavities formed by the vanes to which these straps are attached. In Figs. l and 2, these straps are the inner straps interconnecting vanes 14e and 14g and vanes 14m and 14p and the outer straps interconnecting vanes 14j and 14h and vanes 14n and 14g. The remaining straps are undistorted and, as previously stated, lie in a plane parallel with the plane of the top faces of the anode vanes.

The straps which cross critical load axis 30 may be pushed down rather than pulled up, in which case the straps crossing axis 31 would be pulled up. In other words, if an asymmetryof the type tending to increase the cavity resonator frequency, such as a reduction of strap inductance or capacitance, is introduced at positions displaced from the output coupling means by angles of +45 and +225, it follows that an asymmetry of a type tending to decrease the frequency should be placed at points displaced from the output coupling means by angles of 45 and -225, that is, at angles of 45 displaced from the output axis 20. In this Way, the effects of the asymmetries will not neutralize one another and the output discontinuity will not predominate and fix they orientation of lthe doublet, as it does in a symmetrical anode resonant structure.

If the anode resonant structure were symmetrical except for the output-coupling means, it would be necessary to bend the straps at only a single position displaced fortyiive degrees from the output axis; it is also possible to bend the straps at two diametrically opposite points located either on critical load axis 30 or on critical load axis 31. Because of practical construction asymmetries,

however, it is preferable to bend the straps rat the rfour points just described so as to eliminate any' random asymmetries that may olfset the eifect of the asymmetry purposely introduced.

As previously stated, in some magnetrons containing more than the usual number of asymmetries as a result of imperfections in the manufacturing process, or in magnetrons operating into a large reactive load, the strapping asymmetry or asymmetries introduced intoy the anode block may be positioned at a'critical load angle from the output axis different from forty-five degrees. Moreover, it has been found in practice that the desired suppression of oscillation in the adjacent doublet mode may be accomplished even though the coupling of each doublet to the output coupling means (load) is not exactly equal;

the angle between the output axis 20 and the strap asymn metry has been found to range from about 30 degrees to 60 degrees, depending on the factors abovementioned and on the particular magnetron under consideration. Regardless of the value of the angle between the output axis 20 and the strap asymmetry, axes 30 and 31 will remain at right angles to one another.

For optimum coupling, the asymmetry'or asymmetries should be so introduced that the points of maxima and minima for each doublet are located physically at angles kof 45 degrees with output axis 20.

A shift of 45 electrical degrees corresponds toa shift in the voltage maxima andvminima points of 45 physical degrees, as clearly shown in Fig. 3.

In Fig. 3b, both doublet wave patterns are displaced forty-five electrical degrees from the position shown in Fig. 3a. The instantaneous anode radio frequency potential at output vanes 14g and 14a for the two doublets is now as shown at points E and F. The radio frequency potential at anode vanes 14n and 14p which are displaced 45 physical degrees from anode output vanes 14q and 14a for the first and second doublets is as shown at points A', B' and C', D', respectively, of Fig. 3b, which correspond exactly to the potential existing at points A, B, C and D, respectively, at output vanes 14q and 14a of Fig. 3a. In other words, the physical displacement of the radio frequency potential maxima and minima in degrees is equal to the electrical displacement in electrical degrees.

Inspection of Fig. 3b indicates that the potential difference between output anode vanes 14a and 14g for the first doublet has been reduced from that shown in Fig. 3a while that for the second doublet has increased. More important, however, is the fact that the aforesaid potential difference is now substantially equal for both doublets. This, of course, means that the two doublets are now coupled equally to the output coupling means.

In Fig. 3c, the distribution of anode potential for the 1r or n=8 mode is shown. The pattern 27 has zero potential at each anode vane, as shown at points I and J, so that this pattern is meaningless. The pattern 28 displaced from pattern 27 by 1r/2 radians has a maximum value of potential at each anode vane, as shown at points K and L. It is evident, therefore, that the pi mode is not degenerate. The reorientation of the adjacent doublet mode, therefore, has no eiect upon the operation of the magnetron in the pi mode.

As previously stated, the desired orientation of the two doublets of the adjacent mode may be produced by 6 the introduction of any type of asymmetry in the anode structure in the regions previously discussed, such as a differently shaped tuning finger or echange in any parameter that will change the natural resonant frequency of a single cavity by an amount of theorder of a few percent.

This invention is not limited to the specific embodiment herein illustrated and described but includes such modifications thereof as fall within the scope of the following claims.

What is claimed is:

l. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure concentrically mounted with respect to said cathode, said anode structure containing a plurality of segments defining cavity resonators, output-coupling means in one of said cavity resonators for removing energy from said anode structure, a plurality of electrically-conductive straps interconnecting alternate anode segments, an asymmetry introduced in at least one portion of said straps along at least one critical load axis displaced from an output .axis passing through the center of said output coupling means and the geometrical center of said magnetron by a critical load angle lying substantially within the range of 30 to 60 and dependent upon the number and location of asymmetries in said anode structure.

2. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comlprising a cathode and an anode resonant structure costructure, means including at least one anode segment deformation positioned on an axis displaced from an output axis passing through the center of said output-coupling means and the geometrical center of said magnetron by a critical load angle lying substantially within the range of 30 to 60 and dependent upon the number and location of asymmetries in said anode structure.

3. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure coaxially mounted with respect to said cathode, said anode structure containing a plurality of segments dening cavity resonators, output-coupling means in one of said cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, means including asymmetries introduced in those of said straps which are angularly displaced from said output-coupling means by angles of substantially +45 and +225 only for altering orientation of the doublets of said adjacent mode with respect to said output-coupling means.

4. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure coaxially mounted with respect to said cathode, said anode structure containing a plurality of segments defining cavity resonators, output-coupling means in 'one of said cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, means including asymmetries introduced in those of said straps which are angularly displaced from said output-coupling means by angles of substantially 45 and -225'only for alter ing orientation of the doublets of said adjacent mode with! respect to said output-coupling means.

5. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure co axially mounted with respect to said cathode, said anode structure containing a plurality of segments defining; cavity resonators, output-coupling means in one of said'` cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, means including an asymmetry introduced in said straps which are angularly displaced from said output-coupling means by angles of substantially +45 and +225 for changing in one direction the natural resonant frequency of the resonators located at said +45 and +225 position, and means including an asymmetry introduced in said strap which are angularly displaced from said output-coupling means by angles of substantially 45 and 225 ter changing in the opposite direction the natural resonant frequency of the resonators located at said -45 and 225 position.

6. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure c0- axially mounted with respect to said cathode, said anode structure containing a plurality of segments defining cavity resonators, output-coupling means in one ot said cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, means positioned substantially 45 from said output coupling means for altering the orientation of the doublets of said adjacent mode with respect to said output coupling means.

7. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure concentrically mounted with respect to said cathode, said anode structure containing a plurality of segments detining cavity resonators, output coupling means in one ot said cavity resonators for removing energy from said anode structure, a plurality of electrically conductive straps interconnecting alternate anode segments, said anode structure being substantially symmetrical except for said output coupling means, .an asymmetry introduced in at least one portion of said straps along at least one critical load axis displaced from an output axis passing through the center of said output coupling means and the geometrical center of said magnetron by a critical load angle equal to substantially 45.

8. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure coaxially mounted with respect to said cathode, said anode structure containing a plurality of segments defining cavity resonators,'output coupling means in one of said cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, means including at least one deformation of one of said straps positioned on an axis displaced from an output axis passing through the center of said output coupling means and the geometrical center of said magnetron by a critical load angle substantially equal to 45 for altering the orientation of the doublets of said adjacent mode with respect to said output coupling means.

9. A magnetron capable of operation in a desired mode and in a doublet mode adjacent said desired mode comprising a cathode and an anode resonant structure coaxially mounted with respect to said cathode, said anode structure containing a plurality of segments defining cavity resonators, output coupling means in one of said cavity resonators for coupling energy out of said anode structure, a plurality of electrically-conductive straps interconnecting said anode segments, said anode structure being substantially symmetrical except for said output coupling means, an asymmetry introduced in at least one portion of said strap along at least one critical load axis displaced from an output axis passing through the center of said output coupling means and the geometrical center of said magnetron by a critical load angle equal to substantially 45.

References Cited in the tile of this patent UNITED STATES PATENTS 2,409,913 Tonks Oct. 22, 1946 2,422,028 Martin June l0, 1947 2,445,447 Martin July 20, 1948 2,504,329 Heising Apr. 18, 1950 OTHER REFERENCES Proc. I. R. E., v01. 35,N0.4,Ap1-i1 1947,pp. 361-369. 

